1. Field of the Invention
The present invention relates to a transfer and fixing device, an image forming apparatus using the transfer and fixing device and an image forming method using the image forming apparatus which are used for a printer or a copying machine using an electrophotographic method.
2. Description of the Prior Art
Conventionally, image forming apparatuses which are capable of forming an electrostatic latent image on a photosensitive body, and developing the image using a dry toner and thereafter electrostatically transferring and fixing a toner image on a recording medium have been popularly used. In such image forming apparatuses, because of irregularities of the surface of a paper which constitutes a recording medium, the paper and the photosensitive body do not completely come into contact with each other and an uneven or irregular gap is formed between the paper and the photosensitive body and hence, a transfer electric field is disturbed or Coulomb repulsion occurs between toners thus disturbing the image.
To cope with such a problem, there have been proposed an image forming apparatus which is capable of forming a multiple toner image of the multicolor by eletrostatically overlapping and transferring plurality of toner images to an intermediate transfer body of an endless belt shape, and fusing the multiple toner image on the intermediate transfer body and thereafter simultaneously transferring and fixing the fused multiple toner image to a recording medium so as to obtain a color copy, and an image forming apparatus which is capable of fusing a multiple toner image of the multicolor which is formed on a photosensitive body having an endless belt shape and thereafter simultaneously transferring and fixing the fused multiple toner image to a recording medium so as to obtain a color copy. These color image forming apparatuses which are capable of transferring and fixing simultaneously have an advantage that the transfer of the toner image to the recording medium is performed non-electrostatically and hence, the previously mentioned degrading of image caused by the electrostatic image transfer hardly occurs.
As an improvement of this image forming apparatus, Japanese Patent laid-open No. 19642/1993, Japanese Patent laid-open No. 107950/1993 and Japanese Patent laid-open No. 249798/1993 and the like disclose a method where for further improving the transferability of a toner image formed on the surface of an intermediate transfer body having an endless belt shape or a photosensitive body to a recording medium, the intermediate transfer body or the photosensitive body carrying the toner image and a paper are heated or pressed under a condition that they are closely brought into contact with each other, and thereafter, the toner image is cooled and solidified while holding the intermediate transfer body or the photosensitive body and the recording medium in an overlapped manner, and then, the recording medium to which the toner image is transferred and fixed is peeled off from the intermediate transfer body or the photosensitive body.
In the image forming apparatus adopting the above method, when the cohesion between toners becomes greater than the adhering force between the toner and the medium, the toner image is peeled off from the intermediate transfer body or the photosensitive body and hence, the occurrence of a so-called offset which remains a part of the toner image on the intermediate transfer body or the photosensitive body can be prevented. Accordingly, it becomes possible for the image forming apparatus to require no oil and the transferring efficiency of the toner is enhanced thus achieving a favorable color balance of the image. Still furthermore, by using the intermediate transfer body or the photosensitive body having a smooth surface and solidifying the toner image by way of such a smooth surface, a high quality image which is excellent in luster and transparency of the toner and thus exhibits a high-grade feeling can be obtained.
In the image forming apparatus adopting the above method, however, a large number of wrinkles or creases are formed on the recording medium parallel to a recording medium transferring direction after heating and pressing operations by the transfer and fixing device and it gives rise to a phenomenon that the toner image is partially peeled off from the intermediate transfer body or the photosensitive body before the toner image is cooled and solidified. When this phenomenon occurs, in addition to the occurrence of wrinkles on the recording medium, the image quality is remarkably degraded.
FIG. 9(a) is a plane view showing the condition where the wrinkles are generated on the recording medium after heating and pressing using a conventional transfer and fixing device and FIG. 9(b) is a cross-sectional view taken along a line B--B of FIG. 9(a).
As shown in FIG. 9(a), a large number of wrinkles 9 may occur in parallel to the recording medium conveying direction A on the recording medium P after heating and pressing using the conventional transferring and fixing device. When the wrinkles 9 occur, as shown in FIG. 9(b), the recording medium P is peeled off from the intermediate transfer body 1 before the toner image t is cooled and solidified and the toner image t is partially solidified under a condition that a part of the toner image t is not closely brought into contact with the intermediate transfer body 1. Accordingly, the surface of the solidified toner image t is formed in an irregular condition where low luster portions C and high luster portions D are present in a mixed form thus the glossiness of the image is lowered and the image quality is drastically degraded.
The wrinkles which are parallel to the recording medium conveying direction A also occur in the conventional fixing device which adopts a roller method. In such a fixing device, as disclosed in Japanese Patent laid-open No. 146806/1996, for example, a nip width at both roller axial direction end portions of a nip region where a pair of fixing rollers press each other is set to be wider than the nip width at the roller axial central portion and hence, the occurrence of wrinkles can be restricted.
FIG. 10 is a schematic view showing the nip region defined by a pair of rollers having resilient body layers respectively.
As shown in FIG. 10, in the nip region N where a heating roller 2 and a pressure roller 3 which are mounted in the fixing device press each other, a resilient layer 3a which is formed on the surface of the pressure roller 3 is subjected to a compressive deformation. When a pair of rollers 2, 3 are rotated in a C direction and a D direction respectively under this condition, to maintain the steady state, a resilient body having the same weight must pass through a portion A (a portion hatched in a rightward and upward direction) which is subjected to the compressive deformation and a portion B (a portion hatched in a leftward and downward direction) which is not subjected to the compressive deformation. Accordingly, the circumferential speed of the portion A where the cross-sectional area thereof is reduced due to the compressive deformation is greater than the circumferential speed of the portion B. Especially, at the portion A, as the part thereof comes closer to the surface of the layer, an amount of increase of the circumferential speed becomes greater. The amount of increase of the circumfereatial speed depends on the magnitude of the compressive deformation and becomes greater as an amount of deformation is increased.
FIG. 11 is a schematic view showing the principle for preventing wrinkles in a conventional fixing device.
As shown in FIG. 11, in a conventional fixing device which adopts a roller method, usually, a heating roller 2 and a pressure roller 3 make their respective roller axial end portions thereof have larger diameter than the diameter of the central portions thereof and hence, an amount of compressive deformation of the rollers at the respective roller axial end portions thereof are made large while an amount of compressive deformation of the rollers at the central portions thereof are made small. Accordingly, the circumferential speed S2 of a recording medium P in the vicinity of both end portions of the nip region is made faster than the circumferential speed S1 of the recording medium P in the vicinity of the central portion of the nip region. Therefore, before entering the nip region, in the vicinity of the roller axial central portion of the recording medium P, a large resisting force R1 which is directed in a direction opposite to a recording medium conveying direction A occurs and a small resisting force R2 occurs in the vicinity of the roller axial end portions. As a result, a moment M directed outwardly works on the recording medium P before it enters the nip region and then the recording medium P enters the nip region while being stretched laterally and hence, no wrinkle occurs on the recording medium P.
In this manner, as described above, the conventional fixing device restricts the occurrence of wrinkles by making the nip width at both roller axial end portions of the nip region where a pair of rollers 2, 3 press each other wider than the nip width at the roller axial central portion.
However, in the image forming apparatus adopting a simultaneous transferring and fixing method, at the time of heating and pressing, in case the length of nip region in a recording medium conveying direction at both roller axial direction end portions of a pair of rollers is made wider than the length of the nip region in a recording medium conveying direction at the roller axial central portion, that is, in case a roller having a construction similar to that of the fixing device shown in FIG. 11 is used, it is impossible to restrict the occurrence of wrinkles which are parallel to the above-mentioned recording medium conveying direction and rather worsens the situation and hence, the image of high quality cannot be obtained.
FIG. 12 is a schematic view showing the conventional image forming apparatus adopting the simultaneous transferring and fixing method.
As shown in FIG. 12, this image forming apparatus comprises an image reading device 30 which reads image information, an image processing device 40 which processes the read image information, an intermediate transfer body 1 which is an image carrier of an endless belt shape capable of carrying out a circulation movement in an arrow direction A, a toner image forming device which includes photosensitive bodies 4_1, 4_2, 4_3, 4_4 which are respectively registered to black, yellow, magenta, and cyan, a light beam scanning device 20 which forms toner images on respective photosensitive bodies 4_1, 4_2, 4_3, 4_4 based on the image information, and developers 11_1, 11_2, 11_3, 11_4 which store toners of black, yellow, Magenta and cyan, transfer devices 15_1, 15_2, 15_3, 15_4 which transfer toner images formed on the photosensitive bodies 4_1, 4_2, 4_3, 4_4 to the intermediate transfer body 1, a heating part H which fuses the toner images formed on the intermediate transfer body 1, a transfer and fixing device T which simultaneously transfers and fixes the toner images formed on the intermediate transfer body 1 to a recording medium P supplied from a tray 6, a cooler 7 which cools and solidifies the toner image, rollers 5_1, 5_2 which supports the intermediate transfer body 1, a tension roller 5_3 which gives a given tension force to the intermediate transfer body 1, and a peeling-off roller 5_4 which peels off the recording medium P from the intermediate transfer body 1.
The transfer and fixing device T includes a heating roller 2 and a pressure roller 3 and these two rollers heat and pressurize the intermediate transfer body 1, the toner image and the recording medium P at a nip region where these two rollers press each other. In the transfer and fixing device T, the intermediate transfer body 1, the toner image and the recording medium P are joined and are heated and pressurized in a closely contacted condition, and powdery toner is formed in a fused condition and respective toner particles are fused to form a sheet of film. Here, for assuring an efficient heat transfer to the toner image, the intermediate transfer body 1 and the recording medium P must be closely brought into contact with each other. If the contact is insufficient and air infiltrates into some places, the heat capacity varies depending on the places where air is present and the places where air is not present so that the toner image is not uniformly fused thus giving rise to irregularities on transfer and fixing or a so-called offset phenomenon and eventually the deterioration of the image quality. To enhance the degree of close contact between the intermediate transfer body 1 and the recording medium P while sandwiching the toner image therebetween, a resilient body layer is formed on the intermediate transfer body 1. Furthermore, the heating roller 2 and the pressure roller 3 are also provided with resilient body layer on the surface thereof for assuring an application of uniform pressure.
FIG. 13 is a schematic view showing the manner how wrinkles occur on a recording medium in an image forming apparatus which adopts a conventional synchronous transfer and fixing method.
Usually, the heating roller 2 and the pressure roller 3 have both respective roller axial end portions thereof supported and hence, when the rollers press each other, portions of these rollers located in the vicinity of the central portions of these rollers are deflected in directions away from each other so that the width of the nip region located in the vicinity of the roller end portions is made wider than the width of the nip region located in the vicinity of the central portions the rollers. That is, an amount of compressive deformation in the vicinity of both roller end portions is greater than an amount of compressive deformation in the vicinity of the central portions of the rollers. Accordingly, the circumferential speed S2 of the recording medium P and the intermediate transfer body 1 in the vicinity of the both roller end portions becomes faster than the circumferential speed S1 of the central portion of the roller. However, the intermediate transfer body 1 is an endless belt and is given a tension by means of a tension roller 5_3 and hence, it receives the least influence from the speed distribution within the nip region due to the compressive deformation of the resilient body layer formed at the roller surface. Furthermore, as in the case of previously mentioned fixing device adopting the roller method (see FIG. 11), the recording medium P receives a moment M directed outwardly before entering the nip region and thereafter enters the nip region while being stretched laterally. However, after passing through the nip region, the portions located in the vicinity of both end portions pass through the nip region much faster than the central portion and hence, the recording medium P receives a moment M' directed inwardly. As a result, a part of the recording medium P is peeled off from the intermediate transfer body 1 and hence, wrinkles occur and uneven brightness of the image are brought about.
The present invention has been made in view of the above drawbacks and it is an object of the present invention to provide an image forming apparatus which adopts a simultaneous transfer and fixing method and is capable of obtaining a high-grade image which has high luster, a favorable balance of color, and an excellent transparency of toner, a transfer and fixing device used for the image forming apparatus, and an image forming method which is capable of forming a high-grade image using the above-mentioned image forming apparatus.